As discussed in detail in U.S. Pat. No. 6,236,033 to Ebbesen et al., the optical transmission of a single, sub-wavelength aperture in a metal film can be strongly enhanced when the incident light is resonant with surface waves, such as surface plasmons, at the film's surface. In order to couple the incident radiation to the surface waves, the surface of the metal film must be provided with a periodic surface topography (i.e. surface features such as grooves or protrusions). The transmission enhancement has great potential in applications which require high optical throughput at high resolution, such as near-field microscopy and high-density optical data storage. This effect has the attractive feature that the wavelength of the resonance can be tuned by the periodicity of the surface features, the angle of incidence of the incident light and the refractive index of a dielectric provided substantially adjacent to the film. See, e.g., U.S. Pat. No. 5,973,316 to Ebbesen et al., U.S. Pat. No. 6,040,936 to Kim et al., U.S. Pat. No. 6,052,238 to Ebbesen et al., U.S. Pat. No. 6,236,033 to Ebbesen et al., and U.S. Pat. No. 6,285,020 to Kim et al. (each of these patents being incorporated herein by this reference and being referred to herein as the “Previous Patents”).
Whether an apparatus comprises enhanced transmission or not through an aperture, there is an additional problem which reduces the practicality of the apparatus comprising a small aperture, namely diffraction. In other words, when light emerges from a small aperture it typically diffracts light in all directions, especially if the aperture has dimension(s) smaller than, or comparable to, the wavelength of the light that is transmitted. As a consequence, the spot size of the light emerging from the aperture increases rapidly and simultaneously the intensity of the light decreases with distance from said aperture. In addition there is no control over directionality. These features reduce significantly the practicality of such an apparatus for many applications (such as high density storage, near-field optical microscopy, optical interconnection) comprising one or more small apertures. For such applications, it would be desirable to have an apparatus which exhibit high transmission, controlled directionality and small optical divergence. While U.S. Pat. No. 6,236,033 to Ebbesen et al. describes enhanced transmission through a single aperture surrounded by periodic surface topography on at least one of the first and second surface of a metal film, no mention is made of the directionality and the divergence of the emerging light.
In relation with the problem of diffraction by small openings or apertures, the inventors have studied the effects of the periodic surface topography on the angular distribution of the light emerging from a single aperture and it has been found that the surface topography can also be used to provide control over directionality and divergence of the beam exiting the aperture. Surprisingly the divergence of the beam has been found much smaller than expected considering the area on the surface from which the light is radiated.